The present invention relates to a sliding sheet for fixing devices and a manufacturing method for the same. More specifically, the present invention relates to a sliding sheet inserted in between an endless belt constituting one member out of two members which form a nip section for fixing operation and a pressure member for pressing the endless belt from an inner surface side toward the other member out of the two members, and to a manufacturing method for the same.
The present invention also relates to a fixing device having such a sliding sheet and to an image forming apparatus having the fixing device.
General fixing devices include one having a sheet (referred to as a “sliding sheet”) inserted in between an endless belt constituting one member out of two members which form a nip section for fixing operation and a pressure member for pressing the endless belt from an inner surface side toward the other member out of the two members so as to reduce the sliding resistance of the endless belt. Further, oil as lubricant is fed to the inner surface of the endless belt in order to reduce friction between the inner surface of the endless belt and the sliding surface (surface in contact with the endless belt) of the sliding sheet.
Conventionally known sliding sheets of this kind include, as shown in FIG. 11, one prepared by impregnating a glass cloth 908 as a base material with PTFE (polytetrafluoroethylene) 907 or a heat-resistant resin and calcinating the glass cloth 908. Since the sliding sheet has a sliding surface 907S with unevenness which reflects the unevenness of the glass cloth 908, the unevenness retains the oil fed to the inner surface of the endless belt and thereby reduces the friction of the sliding sheet with the inner surface of the endless belt (if the sliding surface is flat, the oil is pushed out from a region equivalent to the nip section with pressure, resulting in failure in sufficient reduction of the friction).
There is also known a sliding sheet, as shown in FIG. 12, which is prepared by bonding a glass cloth 808 as a base material to a PTFE sheet 807 obtained by skiving (shaving) a compression-molded PTFE material.
Moreover, it is proposed in JP 2005-3969 A to place a metal wire mesh onto a heat resistant resin sheet (PTFE sheet) and press it in a heated state to cause plastic deformation, so that grid-like unevenness is given to the PTFE sheet. Similarly, in JP 2003-107936 A, it is proposed to press a sharp tip to a polyimide resin to cause plastic deformation so as to gain unevenness.
Moreover, it is proposed in JP 2002-299007 A to print a glass coat on a base material through thick film printing and to calcinate the base material to gain unevenness.
However, it cannot be said that the sliding sheet of FIG. 11 has enough wear resistance. More specifically, the sliding sheet of FIG. 11 has a problem that not only the sliding surface 907S is abraded away and flattened to cause increase in coefficient of friction but also worn powder generated by abrasion is mixed into oil so that apparent viscosity of the oil is increased, resulting in torque increase.
Since the sliding sheet of FIG. 12 has the PTFE sheet 807 prepared by skiving a compression-molded PTFE material, its wear resistance is superior to that of the sliding sheet of FIG. 11 (impregnated with PTFE). However, the sliding sheet of FIG. 12 has a disadvantage that the process for laminating and bonding the PTFE sheet 807 onto the glass cloth 808 is expensive and causes cost increase.
In the methods disclosed in JP 2005-3969 A and JP 2003-107936 A, the glass cloth is disused, which makes it possible to reduce cost. However, in the methods for gaining unevenness by pressing patterns such as embossing or by pressing sharp objects, the unevenness is eventually lost due to heat and pressure of the nip section. Accordingly, the coefficient of friction may increase, and this may cause a problem that the fixing quality cannot be maintained.
In the method disclosed in JP 2002-299007 A, the glass is higher in coefficient of friction than the fluororesin, which may cause a problem of torque increase during start-up i.e., during the time when oil does not yet fully sit on the sliding surface of the sliding sheet.